Many modern electronic circuits or systems require the use of a reference oscillator which is capable of maintaining a relatively precise frequency output over wide environmental conditions. For example, a radio may use a reference oscillator for receiver operations. Here, the radio bases its operation on a carrier frequency that is output by a central control system, such as a base station. The accuracy of an oscillator circuit may depend on a variety of factors. These factors include the component tolerance of a crystal or other part used in the reference oscillator, the effect of temperature, humidity, and other environmental factors such as shock upon the oscillator circuit. Accuracy also depends on the effects of degradation of the oscillator circuit over time, commonly referred to as "aging". Ordinarily, an oscillator circuit requiring a precise frequency output, is built using components having high accuracy tolerances. Accuracies are usually expressed in terms of parts per million or billion with respect to the operating frequency. For example, components, or circuits, characterized by a low part per million (PPM) accuracy is typically much more expensive than those having a higher PPM accuracy. Depending on the application, circuit components are chosen based on their price/performance characteristics.
Regardless of the components chosen, an oscillator circuit will typically have variations in performance because of the environment and other factors. One example, is the degradation in performance due to changes in temperature. Compensation circuits such as that described in U.S. Pat. No. 4,492,933, issued to Grieco on Jan. 8, 1985, for a Temperature Compensation Circuit for Oscillator With Parabolic Characteristics, can be used to improve the accuracy of the oscillator circuit. Other temperature compensating circuits are well known in the art. A temperature compensation circuit usually requires a temperature sensor, and access to information regarding the temperature sensitivity of the oscillator components.
Oscillator circuits also degrade over time because of the effects of aging. The factors affecting age-based accuracy degradation are varied and include cumulative errors resulting from repeated mechanical stresses to the electronic circuit, degradation of materials used within the circuit to affix the components, temperature cycles, vibration, among others. Thus, the effect of aging on a particular oscillator circuit tends to be random and cannot be easily predetermined.
Prior art approaches to the handling of the effects of aging on an oscillator circuit are usually complex and difficult to implement. For receiver type applications, many approaches to compensate for the effects of aging use automatic frequency control which is generally ineffective once the oscillator circuit has degraded outside a particular error detection window. In such cases, the receiver is typically manually tuned or repaired before normal operations can resume.
A radio receiver that depends on access to a particular carrier frequency for proper operation may be inoperable when a reference oscillator, used to determine the frequency of communication signals, has degraded beyond basic error detection capabilities typically associated with automatic frequency control. It is desirable to have a process whereby the radio receiver can self-calibrate automatically to account for such degradation.